\5.1 is a form of surround sound.
Surround sound is a vague term used to describe various representations of multi-channel audio: Dolby ProLogic, Dolby Digital, SDDS, dts 6.1es, Neo:6, Master Audio, older SQ and QS, 10.2 and so on.
Yes, it's typical for 5.1 to be a discrete format, although it's not unheard of for 5.1 to include matrix-format content, too, with Dolby Digital 5.1ex and dts 5.1es being the most popular (back in the day).
I have always heard surround sound as being representitive of a certain type of encoding, it was for a time also called A-B.
I thought the 5.1 format was more similar to 4 channel descrete, where the encoding is 4 entirely separate channels with equal ability to reproduce frequencies.
Whereas surround has the rear channels reproducing select frequencies.
I have one of the old Lafayette 4 channel decoders that can do the SQ format and I think the CD4 format as well. I outta hook it up to a system one day to see if it still works. I have some old SQ albums here as well.